In apparatus which spray or discharge solvent borne coatings or adhesives, since the solvent evaporates off in the area of the nozzle, coating material components and adhesive materials sometimes accumulate thereon. As a solution to this problem, a solvent is sometimes sprayed onto the nozzle from a separate nozzle to prevent drying and adherence of the liquid or adhesive on the end of the discharge nozzle. The solvent is sprayed during interruption or after completion of discharge operations. However, the use of a separately sprayed solvent in this manner often results in solvent drops falling from the nozzle onto the surface to be coated, therefore disturbing the coating on the surface. This problem becomes particularly acute when the spray or injection quantity of the solvent is excessive.
In applications involving the discharge of coating material and/or adhesive onto individual items carried on a conveyor, intermittent discharge is desirable. However, some volatile components carried in the portion of the coating material or adhesive which is located in the flow passage between the dispensing valve and the discharge orifice tend to evaporate during breaks in the discharge operations. This evaporation causes viscosity changes in the coating material or adhesive when it is later discharged, which results in the discharge of varying amounts of liquid onto the successive items.
In other cases, material accumulation on the nozzle end and hardening over time is so severe that the end of the nozzle must be cleaned with a brush.
In cases where hardening occurs during the use of water soluble emulsive coatings or adhesives, harmful organic solvents such as toluene must be used for rinsing and/or cleaning the end of the nozzle. This further complicates the process, due to the harmful nature of the cleaning agent and the requirement to shut down coating operations during cleaning.
To prevent dispersing of harmful solvents within a plant during coating operations, solvent emission collection ducts connected to a fan may be located around the periphery of the application area to remove and collect solvent emissions. Unfortunately, the use of such collection ducts usually creates an air flow which further promotes adherence and hardening of the liquid at the nozzle end.
It is therefore an object of this invention to overcome problems associated with drying and hardening of liquid at a nozzle end of a spraying apparatus, a problem which is particularly acute when the liquid includes volatile components.
It is another object of this invention to eliminate viscosity changes in the coating material or adhesive located between the dispensing valve and the discharge orifice in intermittent coating operations, particularly in cases where the liquid includes volatile components.
It is still another object of the invention to reduce the amount of nozzle cleaning required during the discharge of a liquid which includes volatile components, and further to minimize the stoppage of coating operations required by such nozzle cleaning.